When we’re making a snap judgement about a fact, the mere presence of an accompanying photograph makes us more likely to think it’s true, even when the photo doesn’t provide any evidence one way or the other. In the words of Eryn Newman and her colleagues, uninformative photographs “inflate truthiness”.

Ninety-two students in New Zealand and a further 48 in Canada looked at dozens of “true or alive statements” about celebrities, some of whom they’d heard of and some they hadn’t, such as “John Key is alive”. As fast as they could, without compromising their accuracy, the students had to say whether each statement was true or not. Crucially, half the statements were accompanied by a photo of the relevant celebrity and half weren’t. The take-home finding: the participants were more likely to say a statement was true if it was accompanied by a photo. This was the case for claims about celebrities being alive or dead, but the effect was stronger for unfamiliar celebrities.

Another study with 70 New Zealand undergrads was similar but this time uninformative photos accompanied obscure general knowledge facts. For example, “Macademia nuts are in the same evolutionary family as peaches” was presented alongside a photo of macadamia nuts that provided no clues as to the veracity of the statement. The same effect was found – the students were more likely to wager that a fact was true when it was accompanied by an uninformative photo.

Why do photos have this truthiness effect? One possibility is that it’s something specifically to do with pictures. To check this, another, similar study was conducted but sometimes celebrity “dead or alive” statements were accompanied by simple verbal descriptions of the celebrities that weren’t helpful for judging the dead-or-alive claim. These verbal descriptions also had a “truthiness” effect, which suggests the truthy effect of photos isn’t unique to them, but must instead have to do with some kind of non-specific process that makes it easier for the mind to seek out confirmatory evidence for the claim that’s being judged. Or, perhaps some feature of the verbal descriptions or photos is being taken as evidence for the attached claim. The researchers can’t be sure: “We speculate that nonprobative photos and verbal information help people generate pseudo evidence,” they said.